Most vehicles in operation today (and many other devices) are powered by internal combustion (IC) engines. Internal combustion engines typically have a plurality of cylinders or other working chambers where combustion occurs. Under normal driving conditions, the torque generated by an internal combustion engine needs to vary over a wide range in order to meet the operational demands of the driver. Over the years, a number of methods of controlling internal combustion engine torque have been proposed and utilized. In most gasoline engines, the output of the engine are primarily modulated by controlling the amount of air (and corresponding amount of fuel) delivered to the working chambers. In many diesel engines, the output is modulated primarily by controlling the amount of fuel delivered to the working chambers.
Some approaches seek to improve the thermodynamic efficiency of the engine by varying the effective displacement of the engine. Most commercially available variable displacement engines are arranged to deactivate a fixed set of the cylinders during certain low-load operating conditions. When a cylinder is deactivated, its piston typically still reciprocates, however neither air nor fuel is delivered to the cylinder so the piston does not deliver any power during its power stroke. Since the cylinders that are “shut down” don't deliver any power, the proportionate load on the remaining cylinders is increased, thereby allowing the remaining cylinders to operate at an improved thermodynamic efficiency. The improved thermodynamic efficiency results in improved fuel efficiency.
Typically, a variable displacement engine will have a very small set of available operational modes. For example, some commercially available 8 cylinder variable displacement engine are capable of operating in a 4 cylinder mode in which only four cylinders are used, while the other four cylinders are deactivated (a 4/8 variable displacement engine). Another commercially available variable displacement engine is a 3/4/6 engine which is a six cylinder engine that can be operated with 3, 4, or 6 active cylinders. Of course, over the years, a variety of other fixed cylinder set variable displacement engines have been proposed as well, with some suggesting the flexibility of operating with any number of the cylinders. For example, a 4 cylinder engine might be operable in 1, 2, 3, or 4 cylinder modes.
Another engine control approach that varies the effective displacement of an engine is referred to as “skip fire” engine control. In general, skip fire engine control contemplates selectively skipping the firing of certain cylinders during selected firing opportunities. Thus, a particular cylinder may be fired during one firing opportunity and then may be skipped during the next firing opportunity and then selectively skipped or fired during the next. In this manner, even finer control of the effective engine displacement is possible. For example, firing every third cylinder in a 4 cylinder engine would provide an effective displacement of ⅓rd of the full engine displacement, which is a fractional displacement that is not obtainable by simply deactivating a set of cylinders.
In general, skip fire engine control is understood to offer a number of potential advantages, including the potential of significantly improved fuel economy in many applications. Although the concept of skip fire engine control has been around for many years, and its benefits are understood, skip fire engine control has not yet achieved significant commercial success in part due to the challenges it presents. In many applications such as automotive applications, one of the most significant challenges presented by skip fire engine operation relates to NVH (noise, vibration & harshness) issues. In general, a stereotype associated with skip fire engine control is that skip fire operation of an engine will make the engine run significantly rougher than conventional operation.
Co-assigned U.S. Pat. Nos. 7,577,511, 7,849,835, 7,886,715, 7,954,474, 8,099,224, 8,131,445, 8,131,447 and other co-assigned patent applications describe a new class of engine controllers that make it practical to operate a wide variety of internal combustion engines in a skip fire operational mode. Although the described controllers work well, there are continuing efforts to further improve the technology and/or to provide alternative approaches to implementing such control. The present application describes a variety of arrangements that can be used to determine and/or control the firing fraction of an engine operating in a skip fire operational mode.